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Re: [TowerTalk] Welding re-bar, preventing corrosion

To:, TowerTalk <>
Subject: Re: [TowerTalk] Welding re-bar, preventing corrosion
From: Red <>
Date: Fri, 07 Jul 2006 22:46:38 -0500
List-post: <>

Rick Karlquist wrote:

>Let me make sure I understand this correctly.  A piece of rebar
>poking out of the concrete below grade will rust.  A piece of
>steel poking out above grade will not rust?  Why is this?  
Both will rust, outside the concrete.  The alkalinity of the concrete 
will delay corrosion of the steel inside the concrete, but this 
protection will be lost if there is an opening below ground that permits 
exchanging ions with the ground.  The alkalinity of the concrete is 
neutralized faster through an opening below grade where water, bearing 
dissolved compounds, is exchanged between the ground and the interface 
of concrete and steel.  Complete closure prolongs the protection.  I 
don't know for how long, if it is measured in years, decades, or centuries.

It probably doesn't make much difference if the soil is strongly 
alkaline.  It will make a difference in the life of the steel if the 
soil is acidic.  Acid attacks iron.

I'm not sure what effect corrosion of the re-bar or other steel members 
will have on the integrity of a tower base.  The products of corrosion 
occupy greater volume than the original steel and will exert great 
pressure on the concrete, which is not particularly strong in tension.  
That may crack the concrete.  Continued corrosion will weaken the steel, 
thus defeating the purpose of having it there. The base may or may not 
fail because of either of these effects.  However, I'd rather not take 
that chance.

Above ground, rust inhibiting paint will slow the exterior rusting and 
there is less exchange of water and dissolved matter between the 
steel-concrete interface and the ground that attacks the steel inside 
the concrete.

>This brings up the question of anchor bolts:  galvanized or black
>steel?  I notice that UST supplies non-galvanized bolts.  However,
>every streetlight I've seen uses galvanized bolts.  According to
>what you say, the black steel is OK on account of it being above
>grade.  Right?  I noticed that the UST bolts are already rusty
>when you get them from UST, not to mention what happens after the first
>rain.  Any advice here?
Sorry, I haven't researched the black steel characteristics.  All of my 
experience with steel, other than re-bar, in towers and tower bases has 
been with galvanized steel.

Perhaps someone else will contribute better information for all of us.

73 de WOØW


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